The invention relates to an optical filter including a transparent substrate having thereon a multilayer coating with low thermal emissivity, high light-transmission, and defined color coordinates.
U.S. Pat. No. 3,682,528 discloses an infra-red interference filter in which there is provided on one side of a transparent substrate, a coating having a first and a second double layer, each of which contains a first layer that is formed by a metal such as nickel and a second layer of silver. Between two double coatings a layer of a selected, transparent dielectric material, e.g., magnesium fluoride or titanium dioxide, is provided whose optical thickness amounts approximately to a quarter- wavelength, in the case of a selected reference wavelength. In the visible range of 400 to 700 nm, the thickness of each of the silver layers is between 55 and 130 .ANG.angstroms. Each of the two first layers has a thickness greater than a minimum of about 5 .ANG.angstroms with a maximum determined by the fact that the transparency in the application of the prime coating may vary by about 2%, so that the transparency of the filter is not markedly impaired.
This known multi-layer coating relates to an optical filter whose silver coatings are extremely thin, but nevertheless act substantially as solid metal plates and have very roughly the same optical properties as solid silver, the filter being suitable for repeatable industrial manufacture and having good stability, especially against moisture and abrasion.
U.S. Pat. No. 3,034,924 discloses a method for the preparation of a protective coating for optical purposes which consists of metal oxides deposited from a vapor on the substrate, e.g. magnesium oxides, and rare earth metals simultaneously deposited thereon from a vapor. This known method permits the application of thin oxide coatings on the basis of oxides and/or oxidizable starting materials, or the economical application of a plurality of coatings of different materials, especially metals and rare earth metals, such that the layers remain partially absorption-free.
Lastly, U.S. Pat. No. 4,965,121 discloses a combination of coatings for glass windows to reduce transmission of solar radiation, which includes two silver layers, the one layer of which has a lesser thickness than the other layer, and in which layers of dielectric material, of magnesium oxide for example, are disposed adjacent the two silver layers. The glass thus provided with five layers is said to be especially suitable as a windshield for automobiles, since the coating packet can have a largely neutral color.
Low-e coatings have generally an emissivity of about 0.1 and a light transmission of about 83%, the emissivity determining the coefficients of heat transmission "k" of window glazing. If the emissivity decreases from 0.1 to 0.04, for example, the k coefficient would be reduced by about 15%, i.e., the thermal energy loss through a window would thus be reduced by about the same percentage.